1. Field Of The Invention
The present invention is directed to the cryogenic purification of industrial by-product hydrogen streams to recover a high purity hydrogen product. More specifically, the present invention relates to the use of a second hydrogen feed stream containing, as impurities, non-readily condensible compounds having boiling points below that of methane to alter the physical properties of the flashed condensate, thus enhancing refrigeration for the process and providing maximum recovery of purified hydrogen.
2. Description Of The Prior Art
Refineries and petrochemical plants employ many processes which produce by-product hydrogen. Thus, many methods are known in the prior art for purifying such by-product hydrogen by cryogenic means to produce high purity hydrogen that can be used for hydrocracking and hydrotreating petroleum materials. Prior art methods for hydrogen purification by cryogenic means have historically compressed and combined the various hydrogen containing feed streams and cooled such combined streams by heat exchange utilizing a series of separations so that the liquid condensate is cooled no further than necessary. The hydrogen is cooled to a low enough temperature so that sufficient impurities are condensed out and the hydrogen meets the required purity specification.
The refrigeration in these various cryogenic processes can be a separate, external refrigeration system as disclosed in Knapp, U.S. Pat. No. 3,626,705, and Meisler, U.S. Pat. No. 3,628,340, or it may be provided by the reduction in pressure of the liquid condensate to cause it to flash to a lower temperature, as exemplified by Bolez, U.S. Pat. No. 3,359,744, who also combines part of the product hydrogen with the liquid condensate to provide additional refrigeration of the system, or it may be an expander, as shown in Banikiotes, U.S. Pat. No. 3,796,059.
Oftentimes, petroleum processes produce a by-product hydrogen stream that also contains nitrogen, or some other non-readily condensible compound which has a boiling point below that of methane. An example is the by-product hydrogen stream from a fluid bed catalytic cracker. These types of impurities are extremely undesirable in hydrocracking or hydrotreating processes. Thus, the cryogenic purification processes known in the prior art have done one of two things. First, they combine all of the by-product hydrogen streams from various industrial processes into one feed stream containing hydrogen, various hydrocarbons, and the non-readily condensible compounds with boiling points below that of methane. See, for example, the process described in Meisler, U.S. Pat. No. 3,691,779. However, if the feeds are combined, either colder temperatures or an adsorption system is required to remove the non-readily condensible impurities, thus requiring additional energy consumption, or the utilization of the hydrogen product is decreased. In Meisler's combined feed system, an adsorption system is employed to remove the nitrogen remaining after a series of cooling and condensation stages with successively lower temperatures. The additional energy consumption which is required to remove these non-readily condensible compounds so that the product hydrogen can be used as a chemical reactant is very costly and clearly undesirable in today's energy conscious society.
The second alternative known in the prior art is simply not to purify such by-product hydrogen streams, but to use them merely as fuel gas. An example of a cryogenic purification process utilizing only by-product hydrogen streams containing hydrocarbons is the aforesaid Bolez patent. Bolez discloses an autogenous cryogenic process which injects part of the purified hydrogen overhead into the flashed liquid condensate in order to provide additional refrigeration such that the lower temperatures will produce hydrogen of the desired purity. The injected hydrogen reduces the partial pressure of the liquid hydrocarbons, consequently reducing their temperature. While the lowered temperature results in a purer hydrogen product, this occurs only at a significant loss of product hydrogen.
Most of the cryogenic processes known in the prior art have considered only the utilization of a single feed stream. This is clearly disclosed by Meisler, U.S. Pat. No. 3,691,779 in which a multiplicity of feed streams are discussed, viz., the original feed stream and the purified stream used to regenerate the adsorption beds, but in each case they are combined into a single feed stream. While the Banikiotes patent discloses an integrated cryogenic process to purify two separate hydrogen streams to recover a single hydrogen product, the second feed stream is used only as a make-up feed stream in order to insure a constant yield of hydrogen product. It is not used to provide increased refrigeration for the system by altering the physical properties of the flashed liquid condensate, thus resulting in increased output of product hydrogen.
An examination of the prior art therefore discloses the need for a cryogenic process which can utilize all types of by-product hydrogen streams without requiring additional costly purification steps or the sacrifice of a percentage of total hydrogen recovery.